1. Field of the Invention
The present invention relates to a proportional pressure reducing valve, and more specifically to a direct acting proportional pressure reducing valve used for controlling a clutch assembly incorporated in an automatic transmission for an automotive vehicle, for instance.
2. Description of the Prior Art
An example of proportional pressure reducing valves is disclosed in Japanese Published Unexamined (Kokai) Utility Model Application No. 60-142371. In this prior-art proportional pressure reducing valve, a proportional solenoid composed of a coil, a plunger, and a push rod is provided at an end of a spool slidably disposed within a central hole formed in a valve housing, and an output port pressure of the valve is controlled by the spool under balanced condition of an axial force generated by the solenoid and applied to one end of the spool and an output port pressure introduced into a feedback chamber and applied to the other end of the spool. This output port pressure is introduced into a clutch assembly for an automatic transmission for an automotive vehicle to control clutch engagement operation.
When the above-mentioned valve is used to control clutch assembly of the automatic transmission, a high control precision and a high pressure response speed are both required for the valve to improve transmission (speed change) feeling or reduce transmission (speed change) shock.
In more detail, the precision of the clutch pressure control operation is closely related to transmission shock. That is, to reduce transmission shock, it is necessary to first keep the clutch under half engagement condition and then under perfect engagement condition by controlling the hydraulic pressure, after a sufficient rotary force has been transmitted. In order words, in case the precision of the clutch pressure control is not high, the half clutch engagement is not obtained, so that a great transmission shock is generated.
On the other hand, the clutch response speed is determined by a delay time from when a clutch assembly piston begins to move in response to an output pressure of the proportional pressure reducing valve to when clutch disks are engaged with each other to transmit a rotary force. Therefore, it is important to reduce the above-mentioned delay or wasteful time. This delay time is dependent upon pressure loss in the valve and piping system. Therefore, there inevitably exists a lower limit of the opening area of the spool valve in order to reduce pressure loss of the valve.
In the case of a cylindrical port, the opening area S can be expressed as EQU S=.pi..multidot.D.multidot.l (1)
where D denotes the spool diameter, and l denotes a spool opening length. Further, an axial force F required for the proportional solenoid can be expressed as EQU F=1/4.multidot..pi.D.sup.2 .multidot.P (2)
where P denotes output port pressure of the pressure reducing valve.
Therefore, in order to reduce the delay time and to improve the response speed, the opening area must be increased, and therefore it is necessary to increase the opening length l or the spool diameter D. However, when the opening length l is increased, it is necessary to increase the effective stroke of the proportional solenoid, so that there exists a problem in that the solenoid dimensions increase
On the other hand, when the spool diameter is increased, it is necessary to increase the axial force F of the proportional solenoid, so that there exist other problems in that the solenoid dimensions increase and further the solenoid current consumption rate increases. On the other hand, when the proportional solenoid dimensions are restricted, the controllable pressure range may be narrowed.
In summary, in the prior-art valve, the controllable pressure range is inevitably narrowed, when the size and the current consumption of the proportional solenoid are reduced under consideration of mounting space and heat generation, thus resulting in a problem in that the clutch easily slides when the clutch pressure is low and the clutch torque is high. Therefore, the response speed and the controllable pressure range are determined by finding an appropriate point of compromise.
To overcome the above-mentioned problem, other structures such that a two-step spool having the feedback portion whose diameter is smaller than that of the port portion or valves of pilot type have been proposed. In the case of the two-step spool, however, the concentricity of the two different diameter portions must be precise and therefore the manufacturing cost is high. Further, since the spool driving force is small, there exists another problem in that the valve is subjected to external disturbance such as friction or hydraulic pressure.
On the other hand, in the case of the pilot valve, the structure is complicated; the manufacturing cost is high; the size is large, so that there are many factors related to delay time and therefore the response speed is low.